A general description of existing cyclone separators can be found in different books and monographs, for example, in the book Principles of unit operations by Alan S. Foust el al., John Wiley & Sons, 1985. A comprehensive survey of modern industrial air cleaning equipment can be found in the monograph "Separation of particles from air and gases" by Akira Ogava, CRC Press, 1984.
The existing industrial equipment suitable for dry or wet cleaning of polluted air can be generally divided into two groups.
The first group is based on imparting vortical motion to the fluid entering the cleaning vessel and separation of the solid particulate therefrom by virtue of centrifugal force. This group combines mechanical collectors, e.g. ordinary cyclones, mufti cyclones, and rotary flow dust collectors.
The second group is based on filtration of the air or on induced precipitation or gravitational settling of a solid particulate, without, however, involving the fluid within the cleaning vessel into the vortical movement. The industrial equipment relevant to this group includes bag filters and electrostatic precipitators.
The performance of existing particle collectors is evaluated according to the ultimate particle size of the solid particulate which can be separated by a collector.
From this point of view electrostatic precipitators are rather efficient devices enabling separation of very fine dust with a particle size lower that 0.01 micron. However, electrostatic precipitators are very costly; they operate only in the periodical mode and require that the particles to be separated be electrically conductive.
Venturi scrubbers can separate a particulate with particle size between 0.01 and 0.1 micron; however, their intrinsic disadvantage is associated with the necessity for a water supply and special measures for providing for a significant pressure drop required for normal operation of these devices. It can be appreciated that organization of the pressure drop is inevitably connected with additional energy expenses.
Bag filters are less expensive than venturi scrubbers or electrostatic precipitators. They are also capable of separating a particulate solid with particle size between 0.1 and 0.01 micron.
The disadvantage of bag filters is associated with the necessity in frequent maintenance due to the fact that filter fabric rapidly becomes greased. An additional disadvantage of bag filters is their limitation to operate at temperatures which are lower than temperatures of hot gases emitting during many industrial processes.
In contrast to the above mentioned types of separators the mechanical cyclone separators are versatile and low-cost devices which operate in a continuous mode and without the necessity of frequent maintenance measures. These devices have been successfully used for a very long time, both as stand-alone units, or in combination with the other types of separators. In the above mentioned monograph by Ogava there is mentioned, for example, a mechanical cyclone separator devised as early as in 1886.
Known-in-the-art cyclone separators are divided into tangential inlet cyclones and axial flow cyclones, depending on whether the feed enters the vessel tangentially with respect thereto or coaxially with respect to the longitudinal axis thereof In tangential flow cyclones the feed enters the cyclone chamber tangentially near the top thereof and it is involved in a spinning motion once it enters the chamber. The tangential velocity of the particles tends to carry them towards the periphery of the chamber. The spiral motion of the fluid results in some inward radial acceleration of the particle, and simultaneously, gravitational force imparts downward acceleration. The result is a downward movement of separated particles adjacent to the vessel wall and upward movement of the gas free of solids along the central region of the vessel interior.
Unfortunately, there are several problems associated with conventional tangential inlet cyclones. For very small sizes of dust less than 10 micron the energy represented by the tangential velocity is insufficient to overcome the centripetal force of the rotating fluid and the very fine dust can be picked up by the upward rotational flow of the gas.
In order to rectify the above defect the axial flow type cyclones were devised, in which the feed enters the vessel coaxially and rotational flow thereof is created by the guiding vanes. The solid particles are thrown on the vessel wall by the centrifugal force acting in the coaxial space. Unfortunately, the axial inlet flow cyclones have inferior efficiency in comparison with the tangential inlet cyclones due to the weak rotational gas flow organized by the guide vanes. This is why axial inlet flow cyclones are usually used in mufti-cyclones.
The main disadvantage of mechanical cyclone separators, in general, is their inability to separate fine solid particulate from the air, since neither available tangential inlet cyclones nor axial inlet cyclones have been so far effective in removing fine dust particulate, especially with a particle size of 1-10 microns.
There are known attempts to increase efficiency of tangential inlet cyclone separators by providing them with various guiding means similar to those used in axial inlet cyclones.
There is known, for example, a centrifugal separator, disclosed in SU 1468609. This device incorporates a vortex chamber with adjustable peripheral blades for imparting a vortical motion to the dust-laden air passing through the separator. Despite the fact that this separator provides for improved reliability of operation and facilitates its maintenance, it is suitable only for removing dust which is larger than 10 microns.
In SU 975099 there is disclosed an aerodynamic cyclone for gas scrubbing, comprising a cylindro-conical body, tangent gas inlet and axial offtake. Within the body there is provided a plurality of bent blades which stabilize vortical motion of the gas and improve evacuation of the solid particulate therefrom via the blade louvre grid towards annular space adjacent the inner surface of the body. The shortcoming of this cyclone separator lies in the fact that it is suitable for separation of particles only larger than 5 microns and is not efficient for separation of fine dust with particle size 1-2 microns.
In conclusion, it should be emphasized that despite the fact that different mechanical separating devices in general and cyclone separators in particular have been known for a long time the problem of cost effective and efficient separation of fine dust is still a vital one and there is still a need for a new and improved device which will ensure efficient and economical separation of fine dust from the air.